In conventional thermal power plants, air combustion systems equipped with an air combustion boiler adapted to burn fuel using air are in the mainstream. On the other hand, thermal power generation by means of an oxygen combustion system has been proposed as a method which makes it easy to recover carbon dioxide (CO2) which is one of the causative substances of global warming and emitted in the largest amount in industrial activities.
In relation to the oxygen combustion system, a method has been disclosed which extracts part of flue gas from an upstream side of an FGD (Flue Gas Desulfurization) equipment after the flue gas is discharged from a boiler and subjected to dust removal, mixes the extracted flue gas with oxygen-rich gas produced by an air separator, and supplies the resulting mixed gas to the boiler together with fossil fuel such as coal (see, for example, Patent Literature 1). Since the system does not supply combustion air with a high nitrogen content to the boiler, CO2 concentration in the flue gas is increased, allowing CO2 to be separated efficiently from the flue gas.
On the other hand, with a coal-burning boiler which uses coal as a fuel, since sulfur oxides (hereinafter referred to as SO2 for short) such as SO2 is produced when a sulfur component contained in the coal is oxidized in the boiler, a large quantity of SO2 is contained in the flue gas. Therefore, the SO2 needs to be removed by the FGD equipment before the flue gas is released to the atmosphere, but the larger the amount of flue gas, the higher the cost for desulfurization.
In contrast, with the oxygen combustion system according to Patent Literature 1, since the flue gas extracted on the upstream side of the FGD equipment is recirculated, it is possible to reduce the amount of flue gas to be treated by the FGD equipment and thereby curb the cost for desulfurization of the flue gas. Such an oxygen combustion system can be put into operation by additionally installing an air separator, oxygen supply line, flue gas recirculation line, and the like while still using an air combustion boiler of an already-existing air combustion boiler system.